Data storage devices such as disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the actuator arm as it seeks from track to track.
A spindle motor rotates the disk (or disks) at a high speed so that the head essentially flies over the disk surface on an air bearing. When accessing the disk during write/read operations, it is typically important for the spindle motor to maintain the disk at a target rotation speed so as to maintain a target data rate when writing data to the disk and reading data from the disk. Certain disturbances affecting the spindle motor may cause the rotation speed to deviate significantly from the target rotation speed. For example, tilting the disk drive may cause a large underspeed disturbance due to a gyroscopic effect. A linear controller, such as a proportional/integral or PI controller, may be unable to sufficiently compensate for these large disturbances and may even become unstable due to the controller saturating the digital-to-analog converter (DAC) that generates the control signal (e.g., current) applied to the spindle motor.
FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 4 defined by servo sectors 60-6N recorded around the circumference of each servo track. Each servo sector 6i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6i further comprises groups of servo bursts 14 (e.g., N and Q servo bursts), which are recorded with a predetermined phase relative to one another and relative to the servo track centerlines. The phase based servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations. A position error signal (PES) is generated by reading the servo bursts 14, wherein the PES represents a measured position of the head relative to a centerline of a target servo track. A servo controller processes the PES to generate a control signal applied to a head actuator (e.g., a voice coil motor) in order to actuate the head radially over the disk in a direction that reduces the PES.